Alkylene glycols, in particular monoalkylene glycols, are of established commercial interest. For example, monoalkylene glycols are being used in anti-freeze compositions, as solvents and as base materials in the production of polyalkylene terephthalates e.g. for fibres or bottles.
The production of alkylene glycols by liquid phase hydrolysis of alkylene oxide is known. The hydrolysis is performed without a catalyst by adding a large excess of water, e.g. 20 to 25 moles of water per mole of alkylene oxide, or with a smaller excess of water in a catalytic system. The reaction is considered to be a nucleophilic substitution reaction, whereby opening of the alkylene oxide ring occurs, water acting as the nucleophile.
Because the primarily formed monoalkylene glycol also acts as a nucleophile, as a rule a mixture of monoalkylene glycol, dialkylene glycol and higher alkylene glycols is formed. In order to increase the selectivity to monoalkylene glycol, it is necessary to suppress the secondary reaction between the primary product and the alkylene oxide, which competes with the hydrolysis of the alkylene oxide.
One effective means for suppressing the secondary reaction is to increase the relative amount of water present in the reaction mixture. Although this measure improves the selectivity towards the production of the monoalkylene glycol, it creates a problem in that large amounts of water have to be removed for recovering the product.
Considerable efforts have been made to find an alternative for increasing the reaction selectivity without having to use a large excess of water. Usually these efforts have focused on the selection of more active hydrolysis catalysts and various catalysts have been disclosed.
Both acid and alkaline hydrolysis catalysts have been investigated, whereby it would appear that the use of acid catalysts enhances the reaction rate without significantly affecting the selectivity, whereas by using alkaline catalysts generally lower selectivities with respect to the monoalkylene glycol are obtained.
Certain anions, e.g. bicarbonate (hydrogen carbonate), bisulphite (hydrogen sulphite), formate and molybdate are known to exhibit good catalytic activity in terms of alkylene oxide conversion and selectivity towards monoalkylene glycol. However when the salts of these anions are used as the catalyst in a homogeneous system, work-up of the reaction product by distillation will pose a problem because the salts are poorly soluble in the glycol and tend to make it semi-solid.
High conversions, good selectivity and a low water/alkylene oxide ratio can be obtained with the process, disclosed in EP-A 0 156 449. According to this document, the hydrolysis of alkylene oxides is carried out in the presence of a selectivity-enhancing metalate anion-containing material, preferably a solid having electropositive complexing sites having affinity for the metalate anions. The said solid is preferably an anion exchange resin, the metalate anions are specified as molybdate, tungstate, metavanadate, hydrogenpyrovanadate and pyrovanadate anions. A complication of this process is that the alkylene glycol-containing product stream also comprises a substantial amount of metalate anions, displaced from the electropositive complexing sites of the solid metalate anion containing material. In order to reduce the amount of metalate anions in the alkylene glycol product stream, this stream is contacted with a solid having electropositive complexing sites associated with anions which are replaceable by the said metalate anions.
It has been proposed to simplify the product recovery procedure by using water-insoluble vanadate and molybdate salts. However, with these metalate anion salts the obtained selectivities are significantly lower than with the water-soluble metalates.
In JP-A-57-139026 there is disclosed a method for reacting alkylene oxide with water in the presence of a halogen type anion exchange resin and in the co-presence of carbon dioxide.
In RU-C-2001901 it is pointed out that the former disclosure has the disadvantage of the formation of carbonates in the reaction mixture which are difficult to separate from the glycols on account of the closeness of their boiling points. This patent publication discloses as its invention the performance of the alkylene oxide hydrolysis reaction in one or a sequence of `extrusion reactor(s)` (continuous reaction), in the presence of `anionite` (anion exchange resin of the quaternary ammonium type) in bicarbonate form and carbon dioxide. The essential difference with the former, Japanese, patent publication appears to be the use of the bicarbonate form of the anion exchanger instead of the halogen form thereof. And yet, the Russian patent does not dispense with the addition of carbon dioxide to the feed.
According to WO 95/20559, the presence of carbon dioxide in the feed is detrimental to the catalytic effect of bicarbonate-exchanged resins of the quaternary ammonium type. In this document there is disclosed a process for the preparation of alkylene glycols wherein an alkylene oxide is reacted with water in the presence of a catalyst composition comprising a solid material having one or more electropositive sites, which are coordinated with one or more anions other than metalate or halogen anions, with the proviso that when the solid material is an anionic exchange resin of the quaternary ammonium type and the anion is bicarbonate the process is performed in the substantial absence of carbon dioxide.
A drawback shared by the conventional anionic exchange resins is their limited tolerance to heat. In practising the process of alkylene oxide hydrolysis according to WO 95/20559 with catalyst compositions based on conventional organic quaternary ammonium ion exchangers it has been found, that under severe reaction conditions (high temperature and/or long service) the selectivity of the conventional resin-based catalysts tends to deteriorate strongly while their activity is even enhanced.
Macrocyclic chelating compounds are known--see for example J. March in Advanced Organic Chemistry; Reactions, Mechanisms and Structures, 4.sup.th Edition 1992, pp 82-87 and 363-364. They have the property of forming complexes with positive ions (cations), although they can also complex neutral molecules. They have a regular organic ring structure containing a plurality of heteroatoms such as oxygen, nitrogen or sulphur. They can be monocyclic, bicyclic or cycles of a higher order. The bonding of cations in these complexes is the result of ion-dipole attractions between the heteroatoms and the positive ions. Thus, the number of the heteroatoms in the molecule determines the binding strength and the size and shape of the cavity determines the ions (or neutral molecules) which can be bound. The macrocycle is called the host and the ion is the guest. Owing to their shape and size, the ability of the host molecules to bind guests is often very specific, enabling the host to pull just one cation or molecule out of a mixture.
The best known macrocyclic chelating compounds are those wherein all or most of the heteroatoms are oxygen, in particular the crown ethers wherein the ring structure is two-dimensional (monocyclic) and the cryptands wherein the ring structure is three-dimensional (bicyclic, tricyclic etc.). When the cavity of the macrocycle is spherical the molecule is called spherand. Other more exotic types are the calixarenes, cryptophanes, hemispherands and pondands.
Crown ethers are usually denoted by their total number of atoms and number of heteroatoms in the ring, plus substituents when present. Examples are 12-crown-4 (I), 15-crown-5 (II) and dicyclohexano-18-crown-6 (III). An example of a calixarene is 4 tert-butylcalix(4)arene (V). ##STR1##
Crown ethers have been used as phase transfer catalysts, i.e. for getting a reactant anion of a hydrophylic compound into the organic phase where it can react with a substrate. For example, a salt like KCN is converted by dicyclohexano-18-crown-6 into a new salt (IV), whose anion CN.sup.- is the same but whose cation is much larger, having its positive charge spread over a large volume and hence much less concentrated. This larger cation is much more attracted to organic solvents. Though KCN is generally insoluble in organic solvents, its crown ether complex is soluble in many of them, making its reactions in the organic phase possible.
In U.S. Pat. No. 4,645,817, equivalent of EP-B 0 159 643, there is disclosed a process for the preparation of a hydroxyl group containing alkoxylation product of an organic carboxylic acid, which process comprises reacting an organic compound containing at least one carboxyl group with an alkylene oxide in an alkoxylation reaction in the presence of a phase transfer catalyst. The catalyst may comprise a basic alkali metal compound which has undergone crown ether complex formation. Here the crown ether is used to phase transfer the basic alkali metal alkoxylation catalyst.